I'd use GADT syntax for this:

{-# LANGUAGE GADTs #-}
data Pair a where Pair :: Eq a => {x::a, y::a} -> Pair a

Sjoerd

On Jul 18, 2013, at 1:05 PM, Christopher Done <chrisdone@gmail.com> wrote:

> Hm, also, with equality constraints you can make the type parametrized, too:
>
> data Pair a' = forall a. (a ~ a', Eq a) => Pair {x::a, y::a}
> equal :: Pair a -> Bool
> equal (Pair x y) = x == y
>
>
> On 18 July 2013 13:00, Christopher Done <chrisdone@gmail.com> wrote:
> Why not this?
>
> data Pair = forall a. Eq a => Pair {x::a, y::a}
> equal :: Pair -> Bool
> equal (Pair x y) = x == y
>
>

> _______________________________________________
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users@haskell.org
> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users